Wireless cellular telephony systems have enjoyed increased popularity in recent years. At various times, the channels that carry voice communications in a cellular telephony system may be idle (i.e., no signal transmission over the channel at a particular time). These unused or idle voice channels may be utilized for other communications such as data communication. In particular, an overlay communication system utilizing unused or idle voice channels for digital data communication is desirable. Cellular Digital Packet Data (CDPD) is such an overlay system which provides mobile datagram service utilizing the idle voice channels of an existing cellular telephony infrastructure. Like standard cellular voice transmissions, the CDPD overlay system transmits data from a plurality of remote subscriber units to base stations which relay the data to external fixed end systems or other CDPD networks. Data communication from the remote subscriber units to the base stations is wireless, and the subscriber units may be movable or stationary.
A consortium of cellular communication carriers prepared and released in 1993 a specification entitled "Cellular Digital Packet Data System Specification." The specification defines a protocol to be used by the industry when transmitting and receiving CDPD data messages over an existing cellular communication system. The protocol specifies that CDPD shall be transmitted at a symbol rate that is an integer multiple of 19.2 KHz. The protocol also specifies that CDPD messages are transmitted in bursts, with each burst having a preamble formed by a dotting sequence of 38 bits followed by a synchronization pattern of 22 bits. Following the preamble is the data sequence comprised of n multiples of 385 bits of data.
In general, demodulation of the received CDPD signals involves first acquiring the signal/burst, then demodulating the so-called steady state portion of the burst. Microslot markers separated by approximately 3.125 microseconds mark the beginning of the burst arrival window. The acquisition process involves detecting the presence of a burst signal, then estimating certain parameters such as burst arrival time and carrier frequency offset. Detection is typically accomplished by comparing some processed portion of the received signal with a detection threshold. If the threshold is improperly set, data bursts will be missed completely (a condition known as "burst miss"), and/or the detector will believe that it has detected a burst when in fact no burst is present (a condition known as "false-alarm").
Two fundamental design parameters in the burst detection process are the burst miss probability (or rate) P.sub.M and the false-alarm probability (or rate) P.sub.FA. Generally, P.sub.M is a function of the signal energy to noise power density ratio (SNR) and P.sub.FA is a function of the noise floor level of a particular communication channel. In a cellular environment, the noise floor is dependant on factors such as amplifier gain, antenna gain, cable losses in front of the CDPD demodulator, and others. Because the noise floor is generally different from site to site, providing a fixed threshold generally results in poor performance.
One way to regulate P.sub.FA is a system known generally as "constant false-alarm rate" (CFAR). In general, a CFAR system attempts to achieve a constant false alarm rate independent of noise level in the receiver. One approach to achieving CFAR is a standard ratio detection scheme in which a correlation value of the preamble portion of the burst is divided by a measure of the signal energy on the preamble portion of the burst. A detection is declared if the calculated ratio exceeds the threshold. An example of how ratio detection may be used in a CFAR system is disclosed in an article entitled "The Effect Of The Frequency Offset On The Probability of Miss In A Packet Modem Using CFAR Detection Method", written by M. R. Solemani and H. R. Girard (IEEE Tran. Communication Theory, Vol. 40, No. 7, July 1992, at pages 1205-1211). The P.sub.FA using ratio detection CFAR can be expressed solely as a function of the threshold setting, and is not dependent on the noise level. However, the P.sub.M using ratio detection CFAR is very sensitive to the signal quality. For mobile CDPD transmitters, signal quality can vary widely depending upon factors such as the particular hardware configuration or the proximity of the mobile unit to the base station.
Thus, there is a need for a CFAR detection scheme having a reduced sensitivity to both noise levels and signal quality, while maximizing burst detection probability (i.e., minimizing P.sub.M).